This invention relates to copper foil for use in manufacturing printed circuit boards, and, especially, to electrodeposited copper foil having a bonding side to be bonded to a polymeric substrate wherein the bonding side is provided with a copper bond-enhancing treatment layer, a copper-arsenic layer deposited on the bond-enhancing layer, and a zinc-containing layer deposited on the copper-arsenic layer. This invention also relates to copper clad laminates made with such foil, and to a process for producing such foil.
Typically, a bonding treatment is effected by subjecting the bonding side, usually the matte side, of a xe2x80x9crawxe2x80x9d electrodeposited copper foil to four consecutive electrodeposition steps. The first consists of the deposition of a microdendritic copper layer which enhances, to a very large degree, the real surface area of the matte side, and thus enhances the foil""s bonding ability. It is followed by electrodeposition of an encapsulating, or gilding, copper layer, whose function is to reinforce mechanically the dendritic layer and thus render it immune to the lateral shear forces of liquid resins occurring in the laminating stage of printed circuit board (PCB) fabrication. The encapsulating step of the treatment is very important, since it eliminates the foil""s tendency toward xe2x80x9ctreatment transferxe2x80x9d and the resulting xe2x80x9claminate stainingxe2x80x9d which can cause a decrease of the dielectric properties of copper-clad laminates. The shape, height, mechanical strength and the number of dendritic microprojections per unit of surface area which constitute the dendritic deposit are the factors instrumental in achieving adequate bond strength of the foil, after all stages of the treatment are completed, when the foil is bonded to a polymeric substrate. The role of the second treatment stage, is to reinforce mechanically, the fragile dendritic layer, by overplating it with a thin layer of sound and strong metallic copper, which locks the dendrites to the base foil structure. Such a dendrites-encapsulation composite structure ought to be characterized by high bond strength and the absence of treatment transfer. The treating parameters which assure just that are relatively narrow. If the amount of the encapsulating, or gilding deposit is too low, the foil will be given to treatment transfer, and, if on the other hand, the gilding layer is too thick, a partial loss of peel strength may be expected. In these first two steps of the treatment the layers are composed of pure copper, in the form of microscopic, spherical micro-projections.
The electrodeposition of the copper bonding treatment is typically followed by deposition of a very thin layer of zinc or zinc alloy, a so-called barrier layer. Its purpose is to prevent direct copper-epoxy resin contact and that is why the zinc-alloy layer (which during lamination is converted to alpha brass), is called the barrier layer. If the bonding treatment composed of copper only is subjected to lamination with epoxy resin systems, it tends to react with amino groups of the resin, at the high laminating temperatures. It, in turn, creates moisture at the foil-resin interface, causing the harmful effect of xe2x80x9cmeaslingxe2x80x9d and possibly delamination. A barrier layer which is plated over all-copper treatment prevents these harmful effects entirely. All three stages of the treatment mentioned above, effected by means of electrodeposition, change the geometry and morphology of the matte side of the foil, assuring the desired mechanical strength of the surface region, as well.
The electrodeposition of the treatment is usually followed by an electrochemical stainproofing which changes the surface chemistry. As a result of this step, the bonding surface is rendered chemically stable. This operation removes weak surface films, which can greatly decrease the adhesion of the solids, and replaces the films with a stable film of controlled thickness, which is responsible for imparting xe2x80x9cdurabilityxe2x80x9d of its properties to the treated surface. The film serves as an undercoat for subsequent bonding. The same stainproofing step protects the opposite shiny side of the foil against atmospheric oxidation.
Contemporary bonding treatments were invented in the early 1970""s and major foil manufacturers are using the same techniques today. The changes that have occurred in the intervening years pertain, by and large, to the composition of the barrier layers, to accommodate technical needs imposed by the emergence of new polymeric dielectric substrates used in the manufacture of PCBs. For example, polyimide substrates introduced to the printed circuit industry fairly recently require a much higher laminating temperature than the epoxy pre-pregs. Consequently, foil manufacturers had to modify a portion of the overall treating processes in order to achieve the composition and performance of barrier layers for the foils that are destined for polyimide applications. Simply speaking, barrier layers on polyimide-grade treatments have to withstand much higher laminating and post-bake temperatures, compared to the treatments destined for epoxy applications. High temperature at the metal-polymer adhesion. A well-designed barrier layer will be self-protected along with the underlying all-copper treatment from heat oxidation and the loss of bond.
Other changes in the technology of the bonding treatment are continuing to occur. For example, some major foil manufacturers build their new treaters with a larger number of individual plating tanks, in order to apply twice the sequence of dendritic deposit followed by encapsulating deposit. Thus, quite often, the first four tanks of the treater are designated and devoted to the application of micro-roughening treatment that consists of a dendritic layer followed by an encapsulation layer and this composite plural layer is repeated twice. This practice is aimed at being able to run the treater at greater speeds, since the initial capital outlay for the construction of the treaters is very high today. Conversely, the larger the number of tanks, with the treater run at more traditional speeds, permits deposition of a greater mass or weight of the treatment to assure acceptable peel-strength on so-called xe2x80x9cdifficult to bond toxe2x80x9d polymeric substrates that aim at higher glass transition temperatures. These substrates, which often are blends that involve multifunctional epoxies, BT resin, polyimide, etc., usually require an increased amount of bonding treatment to assure adequate peel-strength. It should be remembered that aside from its bond-enhancing microstructure, the amount of treatment per surface area of copper foil is also an important factor.
It is estimated that foil manufacturers usually electrodeposit about 5 grams of dendritic deposit per square meter of copper foil and about the same amount of encapsulating deposit, while the mass of the barrier layer is usually about 1 gram per square meter. The amount of treatment deposited on the matte side of the foil, depends on the current density and the plating time as determined by Faraday""s Law. Current density cannot be increased excessively to accommodate higher treater speeds, since the copper foil has to carry the current between the contact rollers and the plating electrolyte.
Excessive currents will cause over-heating of the foil with resulting wrinkling, cosmetic defects, etc. It follows then that the plating time cannot be rendered too short, to accommodate high treater speeds, because the amount of the bonding treatment that has to be deposited is an important consideration. As a result, the treater speeds, although they vary among major foil manufacturers, do not exceed about 100 ft/min. Since a drum-cathode machine operated with a current of about 50,000 amps, produces about 60xe2x80x3 wide, 1 ounce foil at a rate of about 10 feet/min. and twice that speed for xc2xd ounce foil, it follows that one modern treater can process the output of 5 to 8 drum machines, depending on the mix of foil gauges, down times, etc. Aside from a degree of the engineering evolution of the treating machines, and to a lesser degree, the treatment process itself, the very concept and mode of the execution of the bonding treatment process, remains much the same since it was invented.
Techniques for the production of electrodeposited copper foil are well-known, as are techniques for electrodepositing bonding treatments on such foil. For example, U.S. Pat. No. 3,857,681 (Yates et. al.) xe2x80x9cCopper Foil Treatment and Products Produced Therefromxe2x80x9d is an excellent source of know-how for the bonding treatment technology. This patent discloses sequential, plural layer bonding treatment technology which involves the succession of dendritic-powdery deposit, encapsulating deposit and the barrier layer. The barrier layer, according to the Yates et al. patent and as practiced today by major foil manufacturers, consists of a thin layer of zinc alloy distributed uniformly over the micro-profile of the all-copper bonding treatment. This zinc-alloy layer, i.e., barrier layer, can be either plated xe2x80x9cas isxe2x80x9d or created by the heat accelerated diffusion of the metals. Some foil manufacturers electrodeposit a brass barrier layer. Others electrodeposit zinc. In this case, the foil delivered to the laminating plants has the bonding side characterized by a uniform gray color. Since the laminating process involves heat in the course of fabrication of copper clad laminates destined for PCBs, the zinc barrier layer alloys with the underlying all-copper bonding treatment by the process of heat-accelerated diffusion of metals in the solid state. As a result, a layer of yellow, chemically stable alpha brass is thus formed over the surface of all-copper surface. Thus, the Yates patent discloses the formation of the zinc alloy layer by either direct plating of brass, of the diffusion formation of brass. In recent years, major foil manufacturers have introduced minor additional alloying elements to improve the performance of the barrier layer with respect to high temperatures of lamination required in the fabrication of polyimide laminates, high temperature post-bakes, etc.
Metals like nickel, cobalt, tin, etc., may be co-deposited with zinc to achieve these goals.
Unskillful application of the barrier layer in the bonding treatment process can create the effect of under-cutting or xe2x80x9cred-ringxe2x80x9d in the fabrication of PCBs. Red-ring undercut can be easily recognized when the conductor lines peeled back from the laminate surface exhibit pink outside margins quite different from the yellow color of un-attacked brass. Usually the wrong composition of brass alloy is at fault, and the resulting ill effects can be serious. Etching solutions can enter into the foil-epoxy interface, i.e., creep underneath conductor lines, leach out zinc from the brass layer and in effect diminish the functional width of the treated side of the foil that actually bonds track lines to the polymeric substrate. An example: If the track line is 5 mil wide and the ingress of etching solution is a 1 mil on each side, then ⅔ of the potentially available bonding force is lost. Cupric chloride etchant with the addition of hydrochloric acid is the most aggressive promoter of xe2x80x9cred-ringxe2x80x9d undercut. 0.1 mm copper conductor lines (or 4 mil lines) usually have a peel strength lower than {fraction (1/100)} of the peel strength, as tested on 1 cm wide (or {fraction (1/250)} for 1 inch) strip of the clad laminate. This is so, because in the fabrication of the PCBs, the etching solution used to remove un-masked portions of the copper foil, tend to enter underneath conductor lines, leading to xe2x80x9cundercuttingxe2x80x9d and partial dis-bonding. The actual width of the bonded foil-polymer interface becomes narrower than the width defined by the photoresist image, and the peel strength is partially diminished. This obviously undesirable phenomenon can be eliminated. Red ring undercut is a textbook case of the corrosion of metal by mineral acids and etching solutions used in the fabrication of PCBs, through the process of xe2x80x9cdezincification.xe2x80x9d As the name implies, zinc is lost from the alloy, leaving as a residue, or by a process of redeposition, a porous mass of copper having little mechanical strength.
Arsenic is a metal which, when added to brasses, e.g., those containing more than 15 wt % zinc, is most effective in preventing dezincification, and arsenic is added routinely (for the purpose of improving corrosion resistance) to metallurgical brasses, i.e., those produced by melting of metals.
However, in practice zinc/arsenic alloy co-deposition is not known or predicted, except perhaps by the use of cyanide electrolytes, which are expensive and pose complex and even more expensive problems of waste water treatment and compliance with environmental laws.
A primary object of the present invention is a treated copper foil which, when bonded to a polymeric substrate, has an improved bond strength to the substrate and provides good resistance to undercutting by acids used in etching to form electrical circuitry on copper-clad laminates made with the foil.
Another object of the invention is a copper-clad laminate made with the above foil which enables a durable, high strength bond between the foil and its substrate.
Still a further object of the invention is a process for electrodepositing a treatment on a bonding side of an electrolytic copper foil, which treatment provides an improved bond strength and resistance to undercutting.
Other objects and advantages of the invention may become apparent from the following description and from practice of the invention.
The objects of the invention may be achieved by a treated copper foil for use in the manufacture of printed circuit boards, which comprises an electrodeposited copper base foil having a bonding side upon which there is deposited a bonding treatment which includes a copper bond-enhancing treatment layer electrodeposited on the bonding side; a copper-arsenic layer electrodeposited on the bond-enhancing treatment layer; and a barrier layer of zinc or a zinc alloy electrodeposited on the copper-arsenic
The above treated copper foil may be produced by a process for electrodepositing a treatment on a bonding side of a copper base foil, which process comprises electrodepositing a bond-enhancing copper treatment layer on a bonding side of the base foil; co-electrodepositing copper and arsenic on the bond-enhancing treatment from an electrolyte containing copper ions and arsenic ions under electrodeposition conditions effective to deposit thereon a copper-arsenic alloy layer; and electrodepositing zinc or a zinc alloy on the copper-arsenic layer to deposit thereon a zinc or zinc alloy barrier layer.